X-ray metrology systems are often used to measure and characterize small and/or hidden features in various materials. For example, thin film thickness measurement systems often use a technique known as x-ray reflectometry (XRR), which measures the interference patterns created by reflection of x-rays off a thin film. FIG. 1a shows a conventional x-ray reflectometry system 100, as described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997 to Koppel. X-ray reflectometry system 100 comprises a microfocus x-ray tube 110, an x-ray reflector 120, a detector 130, and a stage 140. A test sample 142 having a thin film layer 141 is held in place by stage 140 for the measurement process.
To measure the thickness of thin film layer 141, microfocus x-ray tube 110 directs a source x-ray beam 150 at x-ray reflector 120. Source x-ray beam 150 actually comprises a bundle of diverging x-rays, including x-rays 151 and 152. X-ray reflector 120 reflects and focuses the diverging x-rays of x-ray beam 150 into a converging x-ray beam 160. Converging x-ray beam 160 includes x-rays 161 and 162, which correspond to x-rays 151 and 152, respectively. Converging x-ray beam 160 is then reflected by thin film layer 141 as an output x-ray beam 170 onto detector 130. Output x-ray beam 170 includes reflected x-rays 171 and 172, which correspond to x-rays 161 and 162, respectively.
The reflected x-rays in output x-ray beam 170 are actually formed by reflections at both the surface of thin film layer 141 and at the interface between thin film layer 141 and test sample 142. Detector 130 measures the resulting constructive and destructive interference between the reflected x-rays in output x-ray beam 170 as a reflectivity curve. An example reflectivity curve is shown in FIG. 2. By measuring the fringes in the reflectivity curve, the thickness of thin film layer 141 can be determined, as described in U.S. Pat. No. 5,619,548.
To ensure accurate measurements in any x-ray metrology system, precise x-ray beam shaping within the system is critical. Due to the small dimensions being measured by x-ray metrology systems, any x-ray beams used within such system must be tightly controlled (e.g., focused, collimated, etc.). Therefore, a critical component in many conventional x-ray metrology systems (such as XRR system 100 shown in FIG. 1a) is an x-ray reflector that focuses the x-ray beam onto the sample being measured. An x-ray reflector (such as x-ray reflector 120 shown in FIG. 1a) is typically a doubly curved crystal formed using high-precision machining and grinding operations. This manufacturing process is very time consuming and expensive. Furthermore, incorporation of a doubly curved crystal into an x-ray metrology system requires large crystal mounts that make the incorporation of multiple crystals into a single tool very difficult.
Accordingly, it is desirable to provide a system and method for performing x-ray metrology without using crystal reflectors as a focusing mechanism.